A diesel particulate filter (DPF) which traps particulate matter in the exhaust gas of a diesel engine for vehicles has a particulate matter deposition limit, and when the deposition amount exceeds the limit, it becomes impossible to trap more particulate matter. Therefore, when the deposition amount reaches a predetermined level, a regeneration operation is performed to raise the temperature of the DPF, burn and remove the trapped particulate matter, and enable the DPF to trap particulate matter again.
When the vehicle is running at low speed, the exhaust gas temperature is suppressed low. Since, in the case of a regenerative device which raises the temperature of the DPF by increasing the exhaust gas temperature, regeneration of the DPF cannot be performed when the vehicle is running at low speed, and as a result, a large amount of particulate matter is deposited on the DPF.
Subsequently, if the DPF is regenerated when regeneration of the DPF becomes possible, the DPF may be overheated because a large particulate matter amount deposited on the DPF burns all at once, and heat deterioration of the DPF may occur.
To prevent such a fault, when there is a large DPF particulate matter deposition amount, JP2002-371827A published by the Japan Patent Office in 2002 proposes changing the engine running conditions so that a temperature environment where the particulate matter burns slowly may be obtained.